Metal oxide varistors of the type consisting basically of a sintered zinc oxide composition are being used in increasing numbers as replacements for silicon carbide type voltage surge suppressors and lightning arresters. The zinc oxide varistors, to date, exhibit an instability in that the watts loss value, which is a fairly good indication of the varistor leakage current, continuously increases upon constant subjection to a D.C. or A.C. voltage. This instability phenomena is temperature sensitive, with increasing temperature causing decreased stability. In general, the rate of increase of watts loss at a particular operating voltage will double for every 6.degree. C. increase in temperature. Methods are currently available for rendering the zinc oxide varistors sufficiently stable for continuous operation at practical operating voltages and at temperatures as high as about 70.degree. C. but no effective method to date is available for rendering the varistors sufficiently stable under higher temperature conditions. U.S. Pat. No. 3,928,245 and U.S. Pat. application Ser. No. 836,765 teach effective methods for stabilizing varistors and are incorporated herein. Varistors being used as voltage surge protectors and lightning arresters within medium and power transformers require some means for keeping the varistors bodies at a sufficiently low temperature to ensure adequate stability at the voltage stress to which they are subjected. One method currently employed for cooling the varistor body is a mechanical heat sink which is in thermal contact with the varistor for taking the heat away from the varistor body by the process of heat conduction. Depending upon the power rating of the transformer to be protected the various heat sinks employed can be substantial in both size and cost.
The purpose of this invention is to provide methods and apparatus for keeping the zinc oxides varistors at a relatively uniform temperature independent of ambient temperature and voltage conditions for medium and power transformers of the vapor-cooled type.